U.S. patent application number 16/755858 was filed with the patent office on 2021-07-08 for compressed powder magnetic core, powder for magnetic core, and production methods therefor.
The applicant listed for this patent is DENSO CORPORATION. Invention is credited to Jung Hwan HWANG, Ken MATSUBARA, Masataka MITOMI, Seishi UTSUNO.
Application Number | 20210210258 16/755858 |
Document ID | / |
Family ID | 1000005519575 |
Filed Date | 2021-07-08 |
United States Patent
Application |
20210210258 |
Kind Code |
A1 |
UTSUNO; Seishi ; et
al. |
July 8, 2021 |
COMPRESSED POWDER MAGNETIC CORE, POWDER FOR MAGNETIC CORE, AND
PRODUCTION METHODS THEREFOR
Abstract
A dust core that can significantly reduce the iron loss is
provided. The dust core of the present invention includes soft
magnetic particles comprising pure iron or an iron alloy and a
grain boundary layer existing between adjacent soft magnetic
particles. The grain boundary layer has a compound layer comprising
M.sub.xFe.sub.2-xSiO.sub.4 (0.ltoreq.x.ltoreq.1, M: one or more
types of metal elements that serve as divalent cations). Such a
dust core is obtained by annealing a compact. The compact is
obtained by compression-molding a powder for magnetic cores. In the
powder for magnetic cores, coating layers that coat the surfaces of
soft magnetic particles are each composed of a composite phase in
which spinel-type ferrite represented by M.sub.yFe.sub.3-yO.sub.4
(0.ltoreq.y.ltoreq.1, M: one or more types of metal elements that
serve as divalent cations) is dispersed on a surface of a silicone
resin or inside the silicone resin. The dust core after annealing
exhibits a high specific resistance due to the grain boundary layer
having the compound layer and can reduce both the eddy-current loss
and the hysteresis loss.
Inventors: |
UTSUNO; Seishi;
(Nagakute-shi, JP) ; HWANG; Jung Hwan;
(Nagakute-shi, JP) ; MATSUBARA; Ken; (Kariya-city,
JP) ; MITOMI; Masataka; (Kariya-city, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DENSO CORPORATION |
Kariya-city, Aichi-pref. |
|
JP |
|
|
Family ID: |
1000005519575 |
Appl. No.: |
16/755858 |
Filed: |
October 17, 2018 |
PCT Filed: |
October 17, 2018 |
PCT NO: |
PCT/JP2018/038678 |
371 Date: |
April 13, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B22F 2998/10 20130101;
B22F 2301/35 20130101; H01F 1/14766 20130101; B22F 1/02 20130101;
B22F 3/24 20130101; B22F 2302/45 20130101; B22F 2003/248 20130101;
H01F 41/0246 20130101; B22F 1/0062 20130101 |
International
Class: |
H01F 1/147 20060101
H01F001/147; B22F 1/02 20060101 B22F001/02; B22F 3/24 20060101
B22F003/24; B22F 1/00 20060101 B22F001/00; H01F 41/02 20060101
H01F041/02 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 17, 2017 |
JP |
2017-200706 |
Claims
1. A dust core comprising: soft magnetic particles comprising pure
iron or an iron alloy; and a grain boundary layer existing between
adjacent soft magnetic particles, the grain boundary layer having a
compound layer comprising M.sub.xFe.sub.2-xSiO.sub.4
(0.ltoreq.x.ltoreq.1, M: one or more types of metal elements that
serve as divalent cations).
2. The dust core according to claim 1, wherein the compound layer
coats a surface of the soft magnetic particles in a film-like
shape.
3. The dust core according to claim 1, wherein the compound layer
has a thickness of 10 to 200 nm.
4. The dust core according claim 1, wherein x is larger than 0 and
M is at least one of Mn, Zn, Ni, Mg, and Cu.
5. The dust core according to claim 4, wherein M is Mn and/or
Zn.
6. The dust core according to claim 1, having a specific resistance
of 100 .mu..OMEGA.m or more.
7. A powder for magnetic cores, the powder comprising coated
particles, the coated particles having soft magnetic particles and
coating layers that coat surfaces of the soft magnetic particles,
the soft magnetic particles comprising pure iron or an iron alloy,
the coating layers each comprising a composite phase in which
spinel-type ferrite represented by M.sub.yFe.sub.3-yO.sub.4
(0.ltoreq.y.ltoreq.1, M: one or more types of metal elements that
serve as divalent cations) is dispersed on a surface of a silicone
resin or inside the silicone resin.
8. A powder for magnetic cores, the powder comprising coated
particles, the coated particles having soft magnetic particles and
coating layers that coat surfaces of the soft magnetic particles,
the soft magnetic particles comprising pure iron or an iron alloy,
the coating layers being compound layers comprising
M.sub.xFe.sub.2-xSiO.sub.4 (0.ltoreq.x.ltoreq.1, M: one or more
types of metal elements that serve as divalent cations).
9. A method of manufacturing a powder for magnetic cores, the
method comprising: a resin coating step of coating surfaces of soft
magnetic particles with a silicone resin, the soft magnetic
particles comprising pure iron or an iron alloy; and a ferrite
generating step of generating spinel-type ferrite on a surface of
the silicone resin or inside the silicone resin, the spinel-type
ferrite being represented by M.sub.yFe.sub.3-yO.sub.4
(0.ltoreq.y.ltoreq.1, M: one or more types of metal elements that
serve as divalent cations).
10. The method of manufacturing a powder for magnetic cores
according to claim 9, wherein the silicone resin is a thermosetting
resin; and the resin coating step comprising: an application step
of applying the silicone resin to the surfaces of the soft magnetic
particles; and a curing step of thermally curing the applied
silicone resin after the application step.
11. A method of manufacturing a dust core, the method comprising: a
molding step of compression-molding the powder for magnetic cores
according to claim 7; and an annealing step of heating a compact
obtained in the molding step at 400.degree. C. to 900.degree. C.
Description
TECHNICAL FIELD
[0001] The present invention relates to a compressed powder
magnetic core (referred simply to as a "dust core," hereinafter)
comprising soft magnetic particles and relates also to relevant
techniques.
BACKGROUND ART
[0002] There are a considerable number of products using
electromagnetism, such as transformers, motors, generators,
speakers, inductive heaters, and various actuators. Many of them
use an alternating magnetic field and are usually provided with a
magnetic core (soft magnet) in the alternating magnetic field in
order to obtain a large alternating magnetic field locally and
efficiently.
[0003] Magnetic cores are required not only to have high magnetic
properties in an alternating magnetic field but also to have a less
high-frequency loss (referred to as an "iron loss," hereinafter,
regardless of the material of magnetic core) when used in an
alternating magnetic field. Examples of the iron loss include an
eddy-current loss, a hysteresis loss, and a residual loss, among
which the eddy-current loss is important and should be reduced
because it increases with the square of the frequency of an
alternating magnetic field.
[0004] Existing magnetic cores for reducing the eddy-current loss
include a dust core comprising soft magnetic particles (particles
of powder for magnetic cores) provided with an insulating layer
between adjacent particles (grain boundary). Such dust cores are
used in various electromagnetic devices because of a high degree of
freedom in the shape. In general, the insulating layer of a dust
core is composed of a resin, ceramics, glass, or other similar
material, but the nonmagnetic insulating layer may deteriorate the
magnetic properties (such as saturation magnetic flux density and
permeability) due to the nonmagnetic properties. In this regard,
there are proposed dust cores with insulating layers of spinel-type
ferrite (also simply referred to as "ferrite") that is a magnetic
material, and relevant descriptions are found in the following
Patent Documents 1 to 3.
PRIOR ART DOCUMENTS
Patent Documents
[0005] [Patent Document 1] JP2003-151813A [0006] [Patent Document
2] JP2016-127042A [0007] [Patent Document 3] JP2016-86124A [0008]
[Patent Document 4] JP2009-246256A
SUMMARY OF INVENTION
Technical Problem
[0009] When dust cores are subjected to heat treatment (annealing)
for removal of strain to reduce the hysteresis loss, however, the
insulating layer of ferrite may alter to low-resistance
Fe.sub.3O.sub.4 and/or FeO due to Fe that diffuses from the soft
magnetic particles. Thus, it cannot necessarily be said that dust
cores with ferrite insulating layers have sufficiently high
specific resistance.
[0010] Patent Document 4 proposes a composite soft magnetic
material (dust core) obtained through compression-molding a mixture
of a binder solution (silicone resin) and ZnO powder with
Mg-containing oxide-coated soft magnetic particles and then
performing firing and high-temperature steam treatment thereon.
Grain boundary layers formed here are composed of
[MgZnFe]Fe.sub.2O.sub.4+SiO.sub.2 (see FIG. 5 and [0042] of Patent
Document 4). Also in this case, the specific resistance of the dust
core after the heat treatment is not necessarily high, as in the
cases of Patent Documents 1 to 3.
[0011] The present invention has been made in view of such
circumstances and an object of the present invention is to provide
a dust core of high specific resistance that has a novel insulating
layer different from conventional ones at the grain boundaries of
soft magnetic particles. Another object of the present invention is
to provide relevant techniques thereto.
Solution to Problem
[0012] As a result of intensive studies to achieve the above
objects, the present inventors have successfully obtained a dust
core that ensures a high specific resistance even after heat
treatment, by forming a novel insulating layer different from
conventional ones at the grain boundaries of soft magnetic
particles. Developing this achievement, the present inventors have
accomplished the present invention as will be described
hereinafter.
[0013] Dust Core
[0014] An aspect of the present invention provides a dust core
comprising: soft magnetic particles comprising pure iron or an iron
alloy; and a grain boundary layer existing between adjacent soft
magnetic particles. The grain boundary layer has a compound layer
comprising M.sub.xFe.sub.2-xSiO.sub.4 (0.ltoreq.x.ltoreq.1, M: one
or more types of metal elements that serve as divalent
cations).
[0015] The dust core of the present invention can stably exhibit a
high specific resistance even after exposed to a high-temperature
environment and/or used for a long period of time. For example,
even after heat treatment (annealing) is performed for the purpose
of removing the strain which is introduced into the soft magnetic
particles during compression molding, the insulating property is
less likely to deteriorate, and the high specific resistance of the
dust core can be stably ensured. According to the dust core of the
present invention, therefore, both the reduced eddy-current loss
due to the high insulating property of the grain boundary layer and
the reduced hysteresis loss due to the lowered coercivity of the
soft magnetic particles can be achieved at high levels, and the
iron loss can thus be reliably reduced.
[0016] Powder for Magnetic Cores
[0017] (1) The present invention can be perceived also as a powder
for magnetic cores that is a raw material of the dust core. That
is, the present invention may also be a powder for magnetic cores
that comprises coated particles. The coated particles have soft
magnetic particles and coating layers that coat surfaces of the
soft magnetic particles. The soft magnetic particles comprise pure
iron or an iron alloy. The coating layers each comprise a composite
phase in which spinel-type ferrite represented by
M.sub.yFe.sub.3-yO.sub.4 (0.ltoreq.y.ltoreq.1, M: one or more types
of metal elements that serve as divalent cations) is dispersed on a
surface of a silicone resin or inside the silicone resin.
[0018] When heat treatment (e.g., annealing for removal of strain)
is performed on a compact (dust body) obtained by
compression-molding the powder for magnetic cores of the present
invention, the silicone resin as a first phase and the ferrite
(M.sub.yFe.sub.3-yO.sub.4) as a second phase react with each other
to form the above-described compound layer comprising
M.sub.xFe.sub.2-xSiO.sub.4 at a grain boundary between the soft
magnetic particles. The above-described dust core can thus be
obtained.
[0019] (2) The present invention can be perceived further as a
powder for magnetic cores as below. That is, the present invention
may also be a powder for magnetic cores that comprises coated
particles. The coated particles have soft magnetic particles and
coating layers that coat surfaces of the soft magnetic particles.
The soft magnetic particles comprise pure iron or an iron alloy.
The coating layers are compound layers comprising
M.sub.xFe.sub.2-xSiO.sub.4 (0.ltoreq.x.ltoreq.1, M: one or more
types of metal elements that serve as divalent cations).
[0020] The powder for magnetic cores of the present invention
comprises the soft magnetic particles which are coated with the
compound layers composed of M.sub.xFe.sub.2-xSiO.sub.4 that already
has a high resistance. The dust core composed of the powder for
magnetic cores can exhibit a high specific resistance even without
heat treatment. Of course, even when subsequent annealing for
removal of strain or the like is performed, the dust core can
exhibit a high specific resistance because the compound layers are
excellent also in the heat resistance.
[0021] Method of Manufacturing Powder for Magnetic Cores
[0022] The above-described powder for magnetic cores can be
obtained, for example, by a method of manufacturing as below. That
is, there is provided a method of manufacturing a powder for
magnetic cores. The method includes a resin coating step of coating
surfaces of soft magnetic particles with a silicone resin. The soft
magnetic particles comprise pure iron or an iron alloy. The method
further includes a ferrite generating step of generating
spinel-type ferrite on a surface of the silicone resin or inside
the silicone resin. The spinel-type ferrite is represented by
M.sub.yFe.sub.3-yO.sub.4 (0.ltoreq.y.ltoreq.1, M: one or more types
of metal elements that serve as divalent cations).
[0023] In this case, the powder for magnetic cores can be obtained,
comprising the coated particles in which the coating layer of each
soft magnetic particle is the above-described composite phase. When
heat treatment is further performed on the coated particles, the
powder for magnetic cores can be obtained, comprising the coated
particles in which the coating layers are the above-described
compound layers.
[0024] Method of Manufacturing Dust Core
[0025] The dust core of the present invention can be obtained, for
example, by a method of manufacturing that includes a molding step
of compression-molding the above-described powder for magnetic
cores. When the coating layers of the powder for magnetic cores
(coated particles) are each composed of the composite phase, the
dust core having the above-described compound layer at a grain
boundary can be obtained by performing an annealing step of heating
a compact, obtained in the molding step, at 400.degree. C. to
900.degree. C. Even when the coating layers of the powder for
magnetic cores (coated particles) are each composed of the
composite phase, the annealing step is performed thereby to allow
the hysteresis loss of the dust core to be reduced. It is preferred
to perform the annealing step in a non-oxidizing atmosphere.
[0026] Others
[0027] (1) In the present specification, not only when one type of
a metal element is used but also when plural types of metal
elements are used, they are abbreviated as "M" for descriptive
purposes. When M means a plurality of types of metal elements, "x"
or "y" representing the composition ratio (atomic ratio) indicates
the total of respective metal elements. For example, when M
comprises Mn and Zn, "Mx" means Mn.sub.x1Zn.sub.x2, where x=x1+x2
and 0<x1x2. The parameters "x" in M.sub.xFe.sub.2-xSiO.sub.4 and
"y" in M.sub.yFe.sub.3-yO.sub.4 may be the same or may also be
different.
[0028] (2) Unless otherwise stated, a numerical range ".alpha. to
.beta." as referred to in the present specification includes the
lower limit .alpha. and the upper limit .beta.. Any numerical value
included in various numerical values or numerical ranges described
in the present specification may be selected or extracted as a new
lower or upper limit, and any numerical range such as "a to b" can
thereby be newly provided using such a new lower or upper
limit.
BRIEF DESCRIPTION OF DRAWINGS
[0029] FIG. 1 is a schematic diagram illustrating a generation
process of the compound layer according to the present
invention.
[0030] FIG. 2 is a bar graph illustrating the specific resistance
of dust cores before and after heat treatment according to the
samples.
[0031] FIG. 3 is a set of element mapping images obtained by TEM
observation of the grain boundary layer's cross section of a dust
core according to Sample 1.
EMBODIMENTS FOR CARRYING OUT THE INVENTION
[0032] One or more features freely selected from the present
specification can be added to the above-described features of the
present invention. The content described in the present
specification can be applied not only to the dust core and powder
for magnetic cores of the present invention but also to methods of
manufacturing them. The content regarding a method can also be the
content of a product.
[0033] Compound Layer
[0034] The compound layer is composed of M.sub.xFe.sub.2-xSiO.sub.4
(also simply referred to as "the present compound"). The present
compound has an orthorhombic crystal structure similar to that of
fayalite (Fe.sub.2SiO.sub.4).
[0035] Examples of the metal element (M) contained in the present
compound may include one or more types of Mn, Zn, Ni, Mg, and Cu in
addition to Fe (corresponding to x=0). When M is such a metal
element M, a crystal similar to fayalite is readily generated. In
particular, when 0<x, M may contain at least one of Mn and Zn,
that is, M may consist of Mn and/or Zn. This applies to ferrite
(M.sub.yFe.sub.3-yO.sub.4), which will be described later. The
parameters x and y may be set, for example, as 0<x, y<1,
0.1.ltoreq.x, y.ltoreq.0.7, 0.2.ltoreq.x, and/or y.ltoreq.0.5.
[0036] As the compound layer exists so as to coat the surface
(entire surface in an embodiment) of each soft magnetic particle in
a film-like shape, the dust core can stably have a high specific
resistance. The thickness of the compound layer is, for example,
preferably 10 to 500 nm in an embodiment or 20 to 100 nm in another
embodiment. If the thickness is unduly small, the specific
resistance of the dust core is reduced, while if the thickness is
unduly large, the magnetic properties of the dust core may
deteriorate.
[0037] Silicone Resin
[0038] The silicone resin is a raw material for generating the
compound layer and is a polymer compound having a siloxane bond
(--Si--O--Si-- bond). The silicone resin is preferably a
heat-curable resin (simply referred to as a "thermosetting resin")
because the thermosetting resin is more readily softened after
heating and high interfacial adhesion can be obtained with the soft
magnetic powder.
[0039] Silicone resins include various types such as resin-based,
silane compound-based, rubber-based silicone, silicone powder, and
organically modified silicone oil and a composite thereof. A
resin-based silicone resin for coating, that is, a straight
silicone resin composed only of silicone or a silicone resin for
modification composed of silicone and an organic-based polymer
(such as alkyd, polyester, epoxy, or acrylic), may preferably be
used because the electrical insulating property is enhanced, the
coating (resin coating step) is simplified, etc.
[0040] Specific examples of the silicone resin include 804RESIN,
805RESIN, 806A RESIN, 840RESIN, SR2400, Z-6018, 217FLAKE, 220FLAKE,
233FLAKE, 249FLAKE, SR2402, QP8-5314, SR2306, SR2316, SR2310,
SE5060, SE5070, SE5004, and SR2404, all available from Toray Dow
Corning Silicone Co., Ltd.
[0041] Specific examples of the silicone resin further include
KR251, KR500, KR400, KR255, KR271, KR282, KR311, KR213, KR220,
KR9218, KR5230, KR5235, KR114A, KR169, KR2038, K5206, KR9706,
ES1001N, ES1002T, ES1023, KP64, and KP851, all available from
Shin-Etsu Chemical Co., Ltd. As will be understood, silicone resins
other than these brands may also be used. In an embodiment, a
silicone resin obtained by mixing two or more types of silicone
resins having different types, molecular weights, and functional
groups at an appropriate ratio may be used.
[0042] The silicone resin content is, for example, 0.1 to 1 mass %
in an embodiment or 0.15 to 0.6 mass % in another embodiment with
respect to the soft magnetic powder as a whole (100 mass %/100 mass
parts). If the silicone resin content is unduly small, a necessary
compound layer is not formed, while if the silicone resin content
is unduly large, the magnetic properties of the dust core may
deteriorate. In terms of the powder for magnetic cores as a whole
to which ferrite is given, the silicone resin content is preferably
0.05 to 0.8 mass % in an embodiment or 0.1 to 0.5 mass in another
embodiment.
[0043] Spinel-Type Ferrite
[0044] Ferrite is also a raw material for generating the compound
layer and is a type of iron oxide (ceramics) represented by
M.sub.yFe.sub.3-yO.sub.4 (0.ltoreq.y.ltoreq.1, preferably y=1) with
a metal element (M), Fe, and O, where the metal element (M) serves
as a divalent cation.
[0045] As illustrated in FIG. 1, the ferrite is dispersed on a
surface of the silicone resin or inside the silicone resin at the
stage of the powder for magnetic cores (coated particles).
Accordingly, the composite phase in which the second phase composed
of fine particle-like ferrite is dispersed in the first phase
(matrix phase) composed of a film-like or layer-like silicone resin
is in a state of being formed on a surface of the soft magnetic
particles. When the composite phase is heated, the silicone resin
and the ferrite react with each other, and the substantially
uniform film-like or layer-like M.sub.xFe.sub.2-xSiO.sub.4 is
formed on a surface of the soft magnetic particles or at a grain
boundary of the dust core.
[0046] At the stage of the powder for magnetic cores, even in a
state in which the ferrite is generated (dispersed) on the surface
of the silicone resin, when the powder for magnetic cores is
compression-molded, the ferrite appears to be embedded in the
silicone resin. At least at the stage of the dust core after
annealing, the silicone resin and the ferrite appear to entirely
react to form a substantially uniform compound layer composed of
M.sub.xFe.sub.2-xSiO.sub.4.
[0047] Soft Magnetic Particles (Soft Magnetic Powder)
[0048] The soft magnetic particles comprise pure iron or an iron
alloy. Pure iron powder allows a high saturation magnetic flux
density to be obtained and can readily improve the magnetic
properties of the dust core. When a Si-containing iron alloy
(Fe--Si alloy) powder, for example, is used as the iron alloy
powder, its electrical resistivity is increased by Si, so that the
specific resistance of the dust core can be improved and the
eddy-current loss can be reduced accordingly.
[0049] In an alternative embodiment, the soft magnetic powder may
be Fe-49Co-2V (permendur) powder, sendust (Fe-9Si-6Al) powder, or
the like. The soft magnetic powder may also be a mixture of two or
more types of powders. For example, a mixed powder of pure iron
powder and Fe--Si alloy powder or the like may be used.
[0050] The particle size of the soft magnetic particles can be
adjusted in accordance with the spec of the dust core. The particle
size of the soft magnetic powder is preferably 50 to 300 .mu.m in
an embodiment or 106 to 250 .mu.m in another embodiment. An unduly
large particle size may readily lead to a low-density dust core
and/or an increased eddy-current loss, while an unduly small
particle size may readily reduce the magnetic flux density of the
dust core and/or increase the hysteresis loss.
[0051] As referred to in the present specification, the "particle
size" is indicative of the size of the soft magnetic particles and
specified by sieving. Specifically, the upper limit (d1) and lower
limit (d2) of the mesh size used for the sieving are employed to
indicate the particle size (D), such as d1.about.d2 or
d2.about.d1.
[0052] The soft magnetic powder is obtained, for example, using an
atomization method, a mechanical milling method, a reduction
method, or other similar method. The atomized powder may be any of
a water-atomized powder, a gas-atomized powder, and a
gas-water-atomized powder. The atomized powder (in particular, the
gas-atomized powder) having approximately spherical particles
contributes to a high specific resistance of the dust core because
breakage of the film and other troubles are less likely to occur
when forming or molding the dust core.
[0053] Method of Manufacturing Powder for Magnetic Cores
[0054] (1) Resin Coating Step
[0055] The resin coating step can be performed by applying the
silicone resin to the surfaces of the soft magnetic particles. The
application of the silicone resin can be performed, for example, by
a spray method, an immersion method, or other appropriate method.
It is sufficient for the silicone resin to thinly coat the surfaces
of the soft magnetic particles; therefore, depending on the
viscosity, it is usually preferred to use a resin solution diluted
with a solvent.
[0056] When the silicone resin is a thermosetting resin, the resin
coating step preferably includes an application step of applying
the silicone resin to the surfaces of the soft magnetic particles
and a curing step of thermally curing the silicone resin after the
application step. The curing step can improve the interfacial
adhesion of the silicone resin to the surfaces of the soft magnetic
particles. A drying step may be separately performed after the
application step and before the curing step, or the curing step may
also serve as the drying step. Depending on the type of the
silicone resin, the curing step is preferably performed at
150.degree. C. to 300.degree. C. in an embodiment or 200.degree. C.
to 250.degree. C. in another embodiment for about 30 to 60 minutes.
When the drying step is separately performed, the heating
temperature is preferably set to 60.degree. C. to 150.degree. C. in
an embodiment or 100.degree. C. to 120.degree. C. in another
embodiment.
[0057] (2) Ferrite Generating Step (Ferrite Plating Step)
[0058] The ferrite generating step can be performed, for example,
using an aqueous solution method in which a powder to be treated
(soft magnetic powder) is immersed in a reaction liquid (generation
liquid) (reference: JP2013-191839A), a spray method in which a
reaction liquid is sprayed to a powder to be treated (reference:
JP2014-183199A), a one-liquid method using a reaction liquid that
contains urea (reference: JP2016-127042A), or other similar method.
Any method can be employed to generate the ferrite according to the
present invention.
[0059] The ferrite generating step may be repeated depending on the
film thickness of the ferrite or the like. After the ferrite
generating step, a washing step of removing unnecessary substances
may be performed. The washing step is carried out using an alkaline
aqueous solution, water, ethanol, or other appropriate liquid.
Unnecessary substances to be washed are ferrite particles that did
not contribute to the film formation, chlorine and sodium contained
in the treatment liquid (reaction liquid, pH adjustment liquid),
etc. After the washing step, the powder may be dried. The drying
step may include drying by heating rather than natural drying, and
in this case the powder for magnetic cores can be efficiently
manufactured.
[0060] (3) When manufacturing the powder for magnetic cores
comprising the coated particles coated with the compound layers
composed of M.sub.xFe.sub.2-xSiO.sub.4, it is preferred to further
heat the powder after the ferrite generating step. For example, the
powder obtained in the ferrite generating step is preferably heated
at 400.degree. C. to 900.degree. C. in an embodiment or 600.degree.
C. to 750.degree. C. in another embodiment in a non-oxidizing
atmosphere.
[0061] Method of Manufacturing Dust Core
[0062] (1) Molding Step
[0063] As the powder for magnetic cores is molded at a higher
pressure, a dust core having a higher density and a higher magnetic
flux density can be obtained. Note, however, that an unduly high
molding pressure causes the reduction in productivity and/or an
increase in cost. It is therefore preferred to adjust the molding
pressure to 600 to 1600 MPa in an embodiment or 800 to 1200 MPa in
another embodiment. When a mold lubrication warm high-pressure
molding method (detailed in JP3309970B and JP4024705B) is used,
ultrahigh pressure molding can be performed while extending the
life of the mold.
[0064] (2) Annealing Step
[0065] The annealing step can remove the strain introduced into the
soft magnetic particles in the molding step, and the hysteresis
loss due to the strain is reduced. When using the powder for
magnetic cores comprising the soft magnetic particles (coated
particles) coated with the composite phase, the annealing step
allows the compound layer composed of M.sub.xFe.sub.2-xSiO.sub.4 to
be formed as a grain boundary layer of the dust core.
[0066] Preferably, the annealing step includes, for example,
heating at 400.degree. C. to 900.degree. C. in an embodiment or
600.degree. C. to 750.degree. C. in another embodiment for 0.1 to 2
hours in an embodiment or 0.5 to 1 hour in another embodiment in a
non-oxidizing atmosphere. The non-oxidizing atmosphere as referred
to in the present specification is an inert gas atmosphere, a
nitrogen gas atmosphere, a vacuum atmosphere, or other similar
atmosphere.
[0067] Dust Core
[0068] The specific resistance (in particular, the specific
resistance after the annealing) of the dust core is preferably 100
.mu..OMEGA.m or more in an embodiment, 1000 .mu..OMEGA.m or more in
another embodiment, or 10000 .mu.nm or more in still another
embodiment.
[0069] The dust core can be used, for example, in electromagnetic
devices such as motors, actuators, transformers, inductive heaters
(IH), speakers, and reactors. In particular, the dust core is
preferably used as an iron core that constitutes an armature (rotor
or stator) of an electric motor or a generator.
EXAMPLES
[0070] Dust cores were manufactured using respective powders for
magnetic cores having different coated layers of the soft magnetic
particles. Properties of each dust core were measured and the
structures of the grain boundary layers were observed. The present
invention will be described in more detail with reference to such
examples.
[0071] Manufacturing of Powder for Magnetic Cores
[0072] (1) Soft Magnetic Powder (Raw Material Powder)
[0073] Gas-atomized powder comprising pure iron was used as the
soft magnetic powder. The particle size was 212.about.106 How to
specify the particle size is as previously described.
[0074] (2) Resin Coating Step
[0075] A resin solution was prepared by dissolving a silicone resin
(Shin-Etsu Silicone KR220L available from Shin-Etsu Chemical Co.,
Ltd.) as a thermosetting resin in isopropyl alcohol. The resin
solution was sprayed and applied to the raw material powder which
was being heated (60.degree. C. to 100.degree. C.) and stirred
(coating step). The spray amount was adjusted so that the silicone
resin content would be 0.2 mass % with respect to the raw material
powder (100 mass %).
[0076] The raw material powder after the application step was
heated at 220.degree. C. for 60 minutes in a nitrogen atmosphere.
Thus, the silicone resin layers applied to the surfaces of the soft
magnetic particles were thermally cured.
[0077] (3) Ferrite Generating Step
[0078] The soft magnetic powder after the resin coating step was
stirred while being heated to 130.degree. C. in the air with a
mantle heater, and the ferrite generation liquid (reaction liquid)
was sprayed to the powder. Two types of generation liquids were
prepared as follows. One generation liquid was prepared by
dissolving manganese chloride (MnCl.sub.2), zinc chloride
(ZnCl.sub.2), and iron chloride (FeCl.sub.2) weighed at a molar
ratio of 0.5:0.5:2 in ion-exchange water (Sample 1). The other
generation liquid was prepared by dissolving only iron chloride
(FeCl.sub.2) in ion-exchange water (Sample 2). These generation
liquids exhibited pH 8.
[0079] The powder after the spray treatment with each generation
liquid was washed with pure water (washing step) and dried by
heating to 100.degree. C. (drying step). Thus, the ferrite layer
(second phase) composed of Mn.sub.0.5Zn.sub.0.5Fe.sub.2O.sub.4
(Sample 1) or Fe.sub.3O.sub.4 (Sample 2) was further generated on
the silicone resin layer (first phase) coating each of the soft
magnetic particles (ferrite generating step). In this way, the
powders for magnetic cores (Samples 1 and 2) were obtained, each
comprising the soft magnetic particles (coated particles) having
the coating layers (composite phases) composed of silicone resin
layers and ferrite layers. The ferrite generating step was
conducted also with reference to the description of
JP2014-183199A.
[0080] (4) Comparative Sample
[0081] A comparative sample was also manufactured as a powder for
magnetic cores for which only the ferrite generating step was
carried out using the same generation liquid as that for Sample 1
without performing the above-described resin coating step (Sample
C1).
[0082] Manufacturing of Dust Core
[0083] (1) Molding Step
[0084] The powder for magnetic cores according to each sample was
molded at 1200 MPa using a mold lubrication warm high-pressure
molding method (references: JP3309970B and JP4024705B). Thus, a
compact having a ring shape (40.times.30.times.4 mm) was
obtained.
[0085] (2) Annealing Step
[0086] The compact according to each sample was placed in a heating
furnace and heated at 600.degree. C. for 1 hour in a nitrogen
atmosphere (non-oxidizing atmosphere). Thus, the dust core
according to each sample was obtained.
[0087] Measurement
[0088] The specific resistance of the dust core before and after
the annealing step according to each sample was measured by a
four-terminal method (JIS K7194) using a digital multimeter (R6581
available from ADC Corporation). The obtained measurement results
are illustrated in FIG. 2.
[0089] Observation
[0090] The cross section (mainly the grain boundary layer) of the
dust core according to each sample was observed using a
transmission electron microscope (TEM) and energy-dispersive X-ray
spectroscopy (EDX). Examples of the element mapping images thus
obtained (Sample 1) are shown in FIG. 3.
[0091] Evaluation
[0092] (1) Specific Resistance and Coercivity
[0093] As apparent from FIG. 2, both before and after the heat
treatment (annealing), Samples 1 and 2 exhibit higher specific
resistance than that of Sample C1. In particular, as apparent from
the comparison between those after the heat treatment, it has been
found that the specific resistance of Sample C1 sharply decreases
to less than 100 .mu.nm whereas the specific resistance of Samples
1 and 2 is maintained at a very high state of about 10.sup.5
.mu..OMEGA.m.
[0094] When the dust cores after the heat treatment according to
Samples 1 and 2 are used, therefore, both the reduced eddy-current
loss and the reduced hysteresis loss can be achieved at high
levels, and the iron loss can thus be reliably reduced.
[0095] (2) Structure of Grain Boundary Layer
[0096] As apparent from FIG. 3, the grain boundary layer of Sample
1 is composed of Fe, Si, O, Mn, and Zn, and its thickness is about
70 nm. From the results of the composition analysis, it has been
confirmed that the grain boundary layer is a compound layer
composed of (Mn,Zn).sub.xFe.sub.2-xSiO.sub.4 (x=about 0.2).
Likewise, it has been confirmed that the grain boundary layer of
Sample 2 is a compound layer composed of Fe.sub.2SiO.sub.4
(x=0).
[0097] From the above, it has been revealed that the dust core of
the present invention having the compound layer composed of
M.sub.xFe.sub.2-xSiO.sub.4 at a grain boundary of the soft magnetic
particles can reduce both the eddy-current loss and the hysteresis
loss and can sufficiently suppress the iron loss.
* * * * *